A young staffer for Mayor Bloomberg was left brain-dead early this morning after she and her former Ivy League roommate were mowed down by a hit-run driver in Brooklyn, law-enforcement sources said.

Erinn Phelan, 22 — who works for the mayor’s office as a coordinator with its volunteerism agency — was on life support last night at Kings County Hospital, and sources said she would be removed from the respirator as soon as arrangements were made for the donation of her organs.

Her ex-roommate at Brown University, 23-year-old Alma Guerrero, was in stable condition and expected to be released tomorrow. Guerrero, who attends Brown’s medical school, was in town for the weekend to visit Phelan.

Cops said a green 1993 Acura Legend headed northbound on Flatbush Avenue struck the pair shortly after 4:30 a.m. at Prospect Place in Prospect Heights and did not stop after the collision.

A source told The Post that the women were in the middle of the crosswalk on Flatbush Avenue when they were hit, but it was not clear if they had the light.

The Acura was found abandoned several blocks away on Pacific Street, between Flatbush and Fourth avenues. Cops suspect that the driver may have fled to the nearby Atlantic Avenue subway station and hopped on a train to escape.

An eyewitness confirmed that it was the vehicle involved in the hit-run. Investigators later dusted it for fingerprints.

Records show that the Acura is registered to Cindy Jasmin, 32, of The Bronx. Jasmin’s mother, Nicole, told reporters outside the house in the Wakefield section today that her daughter didn’t own such a car.

“I don’t know about any accident,” the mother said. “She does not live here. She does not own a green Acura.”

Detectives were later seen at Jasmin’s house and were told by a man at the door, “I’ll call you as soon I hear from her.”

Sources said the car was not reported stolen and that at least one witness at the scene told cops he saw a woman behind the wheel of the car.

Police Commissioner Ray Kelly, who went at the hospital tonight to console Phelan’s family, only said, “We have no suspects at this moment.”

He called Phelan’s injuries “very severe.”

Dana Campos, a friend of Guerrero’s who also knows Phelan, described both women as dedicated to public service.

“Erinn was very, very active and very, very involved at Brown,” said Campos, who went to high school with Guerrero in McAllen, Texas, and met Phelan through her.

“They both [participated in a model UN. They were both very involved undergraduates at Brown. They’re both amazing girls, really sweet, loved by everyone.”

Together with Campos, Guerrero founded a non-profit, Frida, that uses art to promote higher education in low-income communities in Texas.

Campos said Guerrero was well enough to speak by phone to her parents, but she said Phelan, who works for NYC Service’s Civic Corps, was “not doing well.”

Mayor Bloomberg urged anyone with information on the hit-and-run to call 311.

“When I spoke to Erinn’s parents, I told them that as a father I can’t begin to imagine what they are going through, but all of our prayers are with their dedicated and idealistic young daughter, who’s helping New York City answer President Obama’s historic call to service,” the mayor said in a statement.